Following a dramatic decline in measles cases due to vaccination, the disease reemerged in 2008, with 604 cases reported to the National Institute for Public Health Surveillance (InVS), followed by 1,544 cases in 2009 and 2,605 cases through June 2010. At the same time, 86 viral strains were identified at the National Reference Center for Measles and Respiratory Paramyxoviridae (CNR) from saliva samples in 2008, 316 in 2009, and 946 through August 2010. The reality of the epidemic is supported by the increase in endemic cases: 0.9 cases per 100,000 inhabitants in 2008 and four in 2010, the spread to all French regions, and the more specific impact on young infants: 4% in 2008 and 9% in 2010, and on individuals aged 20 years or older: 17% in 2008 and 38% in 2010. The majority of cases (82%) obviously occur in individuals who have not been vaccinated against measles. The number of hospitalized cases increased from 18% in 2008 to 34% in 2010. The virus responsible for the outbreak is genotype D4. It first appeared in 2008, then spread in 2009 and 2010, accounting for 19%, 75%, and 99% of characterized viruses. All viruses of this genotype are closely related to the Montreal.CAN/89xD4 strain described in 1989. At the start of the epidemic in France, in the Reims region, a few strains were closely related to the variant identified in England in 2007: MVs/Enfield.GBR/14.07(D4). But today, the vast majority of them (95%) are identical to a variant that caused a small measles outbreak in the Vendée region in the last quarter of 2008: MVs/Montaigu.FRA/43.08(D4). Salivary testing, which was introduced in France following the implementation of mandatory measles reporting, is highly effective: 75% of samples are collected within the first four days of the rash, and RNA testing is positive in 536 (81%) of the 660 saliva samples received by the CNR to date in 2010; 136 (21%) of the saliva samples contain anti-measles IgM, and 18% contain neither RNA nor IgM. (R.A.)